The purpose of generating (or enhancing) the higher order modes is to distribute the energy more evenly throughout the load, and, in particular, to avoid, or at least reduce the occurrence, of uneven temperatures in the load, especially the presence of cold spots at certain locations in the load, usually the center.
The term "mode" is used in the specification and claims in its art-recognized sense, as meaning one of several states of electromagnetic wave oscillation that may be sustained in a given resonant system at a fixed frequency, each such state or type of vibration (i.e. each mode) being characterised by its own particular electric and magnetic field configurations or patterns. The fundamental modes of a body of material to be heated, or of such body and a container in which it is located, are characterised by an electric field pattern (power distribution) typically concentrated around the edge (as viewed in a horizontal plane) of the body of the substance to be heated, or around the periphery of its container when the substance is enclosed by and fills a container, these fundamental modes predominating in a system that does not include any higher order mode generating means. The fundamental modes are thus defined either by the geometry of the container or by the geometry of the body of material to be heated, or to varying degrees by both geometries.
A mode of a higher order than that of the fundamental modes is a mode for which the electric field pattern (again, for convenience of description, considered as viewed in a horizontal plane) corresponds to each of a repeating series of areas smaller than that circumscribed by the electric field pattern of the fundamental modes. Each such electric field pattern may be visualized, with some simplification but nevertheless usefully, as having maxima distributed about a closed loop in the horizontal plane.
The generation or enhancement of such higher order modes can provide more control over the heating of different regions of the substance, and, in particular, render the heating more uniform throughout the substance being heated, compared with the result that would be obtained from the fundamental modes alone.